1. Field of the Invention
The present invention relates to hydraulic fracturing of geologic formations in hydrocarbon wells. More particularly, the present invention relates to tracing the movement and recovery of hydraulic fracturing liquids pumped into oil and gas wells using plural unique oligonucleotides tracing compounds, which correspond with plural fracture stages and zones within a geologic formation.
2. Description of the Related Art
Oil and gas are removed from geologic formations by drilling a well bore from the surface. A well casing is inserted into the well bore, which is then perforated so that oil and gas can flow from the adjacent geologic formation into the well casing. The oil and gas may flow upwardly under natural pressure in the formation, but more commonly they are removed using an artificial lift system, such as the well known sucker-rod pump and surface-mounted pump-jack arrangement. In order to maintain production over an extended period of time, there must be sufficient formation porosity and pressure so that the oil and gas naturally flow from the hydrocarbon bearing geologic formation, through the casing perforations, and into the well casing.
As exploration has expanded into regions where there is insufficient porosity in the oil and gas bearing formations to sustain production, engineers have developed hydraulic fracturing techniques that produce artificial porosity, through which the formation oil and gas can flow into the well casing. Hydraulic fracturing is the fracturing of rock structures adjacent to the well casing perforations using a pressurized liquid pumped down the well casing from the surface. Hydraulic fracturing, or hydrofracturing, also commonly referred to as “fracking”, is a technique in which fresh water is mixed with sand and certain chemicals, and then the mixture is injected at high pressure into a well casing to create small fractures in the formation. This liquid mixture is referred to as fracking liquid. These small fractures enable formation fluids, such as gas, crude oil, and brine water to flow into the well casing. Once the fracking process is completed, hydraulic pressure is removed from the well. The formation rock naturally settles back to its original position, but the small grains of sand, referred to as proppants, hold these fractures open so as to yield the desired artificial porosity. Fracking techniques are commonly used in wells for shale gas, tight gas, tight oil, coal seam gas, and hard rock wells. The fracking process is only utilized at the time the well is drilled and placed into production, but it greatly enhances fluid removal and well productivity over the life of the well.
The sequence of events implemented to place a typical oil or gas well into production generally consists of, drilling the well bore, installing the well casing, perforating the casing, hydrofracturing the hydrocarbon bearing formation, installing an artificial lift system, recovering the hydraulic fracturing liquid, and then producing oil and gas from the well. It is significant to note that the presence of the fracturing liquid in the formation interferes with oil and gas production, and that removal of the fracturing liquid is a technical challenge for operators, and one that must be accomplished promptly, and to a reasonable degree of completion before oil or gas production from the well can commence. This disclosure is primarily concerned with the hydraulic fracturing process and the removal, or other disposition, of the hydraulic fracturing liquid (also referred to herein as “fracking liquid”). The types of wells contemplated herein include common vertical wells and wells in which horizontal drilling is used to traverse a geologic formation so as to increase productivity. In fact, hydraulic fracturing is now commonly employed in wells having horizontal bores through gas producing formations. An example of this is the Barnett Shale formation in north Texas, a region that covers approximately seventeen counties and contains natural gas reserves proven to include 2.5 trillion cubic feet, and perhaps as much a 30 trillion cubic feet of recoverable reserves.
The effectiveness of the hydraulic fracturing process, as well as the flow and disposition of the fracking liquid, is of critical importance to the well operator. Since the fracking process occurs far below the surface and is therefore difficult to monitor, any data that confirms the extent of the fractures or indicates the flow and movement of the fracking liquid is helpful in the operation of that well, and is also informative with regard to similar wells that may be drilled in the same oil field. A technique used to determine the flow and movement of the hydraulic fracturing fluid is called tracing. The tracing process involves placing a marking additive (hereinafter a “tracer”) in the hydraulic fracturing liquid before it is pumped into the well, and then monitoring the fluids subsequently recovered from the well to determine the concentration of the tracer in the well fluids recovered. The concentration of the recovered tracer is compared with the concentration originally pumped into the well, and this is used to estimate the amount of the original fracking liquid that has been recovered. Generally, once a substantial portion of the fracturing liquid has been recovered, the well is placed into production.
Fracturing liquids contain a number of additives and chemicals that are used to facilitate the fracturing process. Among these are specialized sand that is used as a proppant, a thickening or gelling agent that increases viscosity thereby enabling the water to carry the proppant into the fractures, acid used to control pH of the well, a breaking agent that later reduces the viscosity so that the fracturing liquid can be more readily recovered, and numerous other chemical treatment, the details of which are beyond the scope of this disclosure. Some consider a portion of these additives and chemicals to be environmentally questionable, and so the movement of the fracturing liquid is monitored with respect to migration of the fracturing liquids into adjacent formations, possibly including fresh water resources. Thus, it is useful to monitor migration of subterranean fluid movements by detecting the tracer in adjacent oil wells and other access points, such as nearby injection wells and water wells. The fracturing liquids also impede production of oil and gas, and operators take a number of actions to facilitate their removal. This may include chemical treatments to alter the fracture liquids to enhance their removal, and also the addition of flushing liquids to dilute or alter the nature of the fracturing liquids.
Various types of tracers have been employed in hydraulic fracturing liquids. Selection and implementation of a tracer is non-trivial because of the cost constraints and the harsh environment that oil and gas wells present. The tracing material needs to be economically feasible in large scale drilling operations, it must be readily detectable at very low concentrations using commercially available test equipment, and it must survive the extremes of pressure and temperature, and the chemical and biological environment present in an oil and gas well. It is known to use certain chemical tracer compounds, fluorescent dye tracers, radioactive isotope tracers, fluorinated benzoic acid, ionized salts, and certain other chemicals. However, the number of discrete and unique tracers that can be used in a single hydraulic fracturing job is quite limited, and is generally just a handful that would be practicable in a single fracking job. This is a significant limitation because an operator cannot monitor a complex fracking job in detail. Many jobs use only a single tracer, which only enables the tracing of the fracking liquids in total. Some jobs can use individual tracers for a few stages of a fracking job.
Thus it can be appreciated that there is a need in the art for a system and method of tracing hydraulic fracturing liquid that provides greater flexibility, greater detail, and accuracy in a reliable and cost effective manner.